The Pope made the changes by issuing a "Motu Proprio", a decree of his own initiative.

The changes extend the criminal liability of officials and staff, making it possible to indict them for crimes committed outside the Vatican City state.

Under the changes, a list of crimes against children will be specifically referred to in the section of Vatican law dealing with crimes against minors.

They include child prostitution, sexual acts with children and child pornography - crimes punishable by up to 12 years in prison.

The legal changes cover around 5,000 clergy and lay people who work within the Vatican. Sex abuse cases which take place in other countries are dealt with under national legislations.

"The Holy See is finally making up for a serious lag," the international children's rights organisation Terre des Hommes said in a statement, adding that the changes to the law are "of epochal importance".

Pope Francis's decree also imposes tougher penalties on anyone found guilty of disclosing secret church information or documents.

The punishment will increase to two years in jail and a fine for low-importance leaks, to up to eight years in prison if the documents concern the "fundamental interests" of the Holy See.

That measure comes after the Vatican was shaken by the "Vatileaks" affair in which former pope Benedict's butler, Paolo Gabriele, was convicted for stealing personal papal documents and leaking them to the media.

He was pardoned by pope Benedict after being briefly jailed.

Pope Francis also said he would renew the Holy See's commitment to international conventions against organised crime and terrorism.

The Argentine has previously announced moves to tackle the Vatican's murky bank.